The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a semiconductor device having a bulb-shaped recess gate.
As for a typical method for forming a planar gate interconnection line by forming a gate over a flat active region, the current large integration scale of semiconductor devices has caused a channel length to be decreased but an implantation doping concentration to be increased. Accordingly, due to an increased electric field, a junction leakage is generated and thus, it becomes difficult to secure a satisfactory refresh property of a device.
A recess gate process has been suggested to overcome the above described limitations as a method for fabricating a gate interconnection line. The recess gate process forms a gate after etching an active region of a substrate to form a recess pattern. If the recess gate process is applied, the channel length is increased and the implantation doping concentration is decreased. Accordingly, the refresh property of the device can be improved. However, if the recess gate process is employed, a depth of the recess may not be increased enough to completely improve the refresh property. Therefore, a method for fabricating a bulb-shaped recess gate with a rounded and wider bottom portion has been introduced.
FIG. 1 illustrates a typical semiconductor device having a bulb-shaped recess gate. A plurality of device isolation layers 12 are formed in a substrate 11, and the substrate 11 is locally and selectively etched to form a plurality of bulb-shaped recess channels 13.
Each of the recess channels 13 is formed in a symmetrical bulb shape. Accordingly, a threshold voltage variation increasing due to a horn effect may be reduced, and a channel length may be secured.
However, the bulb-shaped recess channels 13 do not secure sufficient spacing between gate patterns. Due to the insufficient spacing between the gate patterns, the gate patterns may cling to each other and thus, a critical limitation may be caused on a device.